1. Field of the Invention
The present invention relates to Schmitt trigger circuits and more particularly to Schmitt trigger circuits utilizing CMOS (complementary metal oxide semiconductor) circuitry.
A Schmitt trigger is a circuit that provides an output signal of either logic-0 or logic-1 in response to an input signal. When the input signal crosses a specified threshold voltage (or current), the output changes to the other logic condition. When the logic condition changes on the output, the threshold voltage is also changed. The difference in threshold voltages is called hysteresis.
The basic operation of a Schmitt trigger circuit is illustrated in FIG. 1. Initially, the output is at logic-0. When the input voltage increases to a high switching threshold voltage VTH+, the output will change states to logic-1. Upon this change, the switching threshold of the Schmitt trigger circuit will be changed from VTH+to a lower threshold VTH-. Therefore, in order to switch back to a logic-0 output, the input voltage must fall below VTH-.
There are at least two applications of the Schmitt trigger. First, when properly used the Schmitt trigger reduces errors in digital systems due to noise on the input signal. The reduction in errors occurs because once the input has switched and the threshold voltage changed, the input voltage must cross both threshold voltages to produce a logic change on the output. If the magnitude of the noise is less than the hysteresis of the Schmitt trigger, then the circuit will not respond to the noise.
In a second application, the Schmitt trigger provides a fast transition on the output, even to slow changes on the input. Therefore, the circuit is useful in waveform generation, such as sine wave to square wave converters, and for pulse generation.
2. Description of the Prior Art
CMOS circuits have been used in the past to fabricate Schmitt triggers. One technique provides a circuit which requires a very small power supply current. This circuit is shown in FIG. 2. It includes an input stage including a CMOS inverter formed of an N-channel field effect transistor N1 and a P-channel field effect transistor P1, and a diode connected transistor N2. The diode increases the switching threshold voltage of the input inverter for the high switching threshold VTH+, and the diode is shorted to obtain VTH-. The control of the diode is by means of an N-channel transistor N3 driven by the output of the Schmitt trigger circuit. This output is derived from a second inverter comprised of transistors P3 and N4 coupled to the output of the first inverter.
The Schmitt trigger thresholds VTH+and VTH- of the circuit of FIG. 2 are determined by the switching thresholds of the transistors N1 and N2. A primary disadvantage of the circuit is that the switching thresholds of the transistors cannot be controlled precisely enough in order to ensure that the circuit will be compatible with TTL (transistor-transistor logic) circuits. Many circuits employ TTL, and it is highly desirable when designing a Schmitt trigger that it be able to be used with TTL circuits. A general requirement for TTL compatibility in a Schmitt trigger circuit is that it must accept anything less than 0.8 volts to cause it to switch to a logic-0 output and must accept anything greater than 2.0 volts to cause it to switch to a logic-1 output. In the circuit of FIG. 2, the combined thresholds of the transistors N1 and N2 may exceed 2 volts, with the result being that the switching threshold for the logic-1 state would be greater than 2 volts (the input voltage must exceed the sum of the thresholds of N1 and N2 in order to switch) and the circuit would therefore not be TTL compatible.